It is known to employ Data Dependant Acquisitions (“DDA”) on a tandem mass spectrometer, such as a quadrupole-Time of Flight mass spectrometer (“Q-ToF”). According to such known techniques, the mass to charge ratios of parent or precursor ions are determined in a survey scan. The quadrupole mass filter then sequentially isolates each individual parent or precursor ion according to its mass to charge ratio and accelerates it into a collision cell to produce product ions. The product ions are then mass analysed in the Time of Flight mass analyser. However, when the parent or precursor ions are isolated the other parent or precursor ions are discarded, leading to a low duty cycle. Furthermore, the parent or precursor ion selection according to this technique results in some bias. For example, if the 20 most intense precursor ions are selected this will bias the data towards the most abundant species.
An improvement on this approach was disclosed in U.S. Pat. No. 6,717,130 (Micromass), wherein precursor ions are not isolated and selected but fragment ions are assigned to parent ions by correlating their detection times to the times as which the parent species eluted from the chromatography column. This technique improves the duty cycle of the instrument and minimises biased acquisitions. However, the technique suffers from specificity limitations since at the point of fragmentation the parent ions are only separated from each other by chromatography.
A known mode of operation of a quadrupole-Time of Flight mass spectrometer is to operate the quadrupole mass filter in a low resolution mode with a transmission window of, for example, 25 Da. The mass to charge ratio range of the ions transmitted by the quadrupole mass filter is then sequentially incremented in steps of approximately 25 Da and in a manner that is not data dependant. Ions exiting the quadrupole mass filter are accelerated into a gas cell and the resulting fragment ions are mass analysed by the Time of Flight mass analyser. The data from each 25 Da window is kept separate for processing. This technique is un-biased in the nature of the acquisition and has an improved duty cycle over devices operating with narrower mass to charge ratio isolation windows. However, the technique has limited precursor ion specificity because any given fragment ion may belong to any of the precursor ions transmitted within a 25 Da window.
It is therefore desired to provide and improved method of mass spectrometry and an improved mass spectrometer.